1. Technical Field
This invention relates in general to nonvolatile memory devices and, in particular, to nonvolatile memory arrays that use electromechanical nanotube technology.
2. Discussion of Related Art
Typical memory devices involve single-bit memory cells that have either an “on” state or an “off” state. One bit of memory storage is determined by either the “on” or “off” condition. The number of bits is dependent directly upon the number of memory cells in a particular memory array. For example, a device, which stores n bits, must have n memory cells. In order to increase the number of memory cells either the overall size of the memory array must increase or the size of each memory element must decrease. Increases in memory cell density have been achieved by improving lithographic techniques that have allowed progress from the production of micron-sized elements to the delineation of nanometer-sized features.
Important characteristics for a memory cell in an electronic device are low cost, high density, low power, high speed and nonvolatility. Conventional memory solutions include Read Only Memory (ROM), Programmable Read only Memory (PROM), Electrically Programmable Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).
ROM is relatively low cost but cannot be rewritten. PROM can be electrically programmed but with only a single write cycle. EPROM has read cycles that are fast relative to ROM and PROM read cycles, but has relatively long erase times and reliability only over a few iterative read/write cycles. EEPROM (or “Flash”) is inexpensive, and has low power consumption but has long (millisecond) write cycles and low relative speed in comparison to DRAM or SRAM. Flash also has a finite number of read/write cycles leading to low long-term reliability. ROM, PROM, EPROM and EEPROM are all nonvolatile, meaning that if power to the memory is interrupted the memory will retain the information stored in the memory cells.
DRAM stores charges on transistor gates that act as capacitors, but its need to be electrically refreshed every few milliseconds complicates system design by requiring separate circuitry to “refresh” the memory contents before the capacitors discharge. SRAM does not need to be refreshed and is fast relative to DRAM, but has lower density and is more expensive relative to DRAM. Both SRAM and DRAM are volatile, meaning that if power to the memory is interrupted the memory will lose the information stored in the memory cells.
As the discussion above indicates, conventional memory solutions fail to possess all the desired characteristics. Existing technologies that are nonvolatile are not randomly accessible and have low density, high cost, and limited ability to allow multiple writes with high reliability of circuit function. Meanwhile, existing technologies that are volatile complicate system design or have low density. Some emerging technologies have attempted to address these shortcomings.
For example, magnetic RAM (MRAM) or ferromagnetic RAM (FRAM) utilizes the orientation of magnetization or a ferroelectric region to generate a nonvolatile memory cell. To obtain nonvolatility, MRAM utilizes magnetoresisitive memory elements involving the anisotropic magnetoresistance or giant magnetoresistance of magnetic multilayer structures. However, both of these types of memory cells have relatively high resistance and low density. A different MRAM memory cell based upon magnetic tunnel junctions has also been examined but has not led to large-scale commercialized devices.
FRAM uses a similar circuit architecture but stores information not in magnetic cells but in thin-film ferroelectric devices. These devices are purported to yield a nonvolatile memory by retaining their electrical polarization after an externally applied electric switching field is removed. However, FRAM suffers from a large memory cell size, and material incompatibility with standard semiconductor CMOS fabrication processes that makes it difficult to manufacture large-scale integrated components. See U.S. Pat. Nos. 4,853,893; 4,888,630; 5,198,994
Another technology having nonvolatile memory is phase change memory. This technology stores information via a structural phase change in thin-film alloys incorporating elements such as selenium or tellurium. These alloys are purported to remain stable in both crystalline and amorphous states, and the fact that these states are electrically distinct allows the formation of bistable switches. Nonetheless, while the nonvolatility condition is met, this technology appears to suffer from slow operations, difficulty of manufacture and reliability problems, and has not reached a state of commercialization. See U.S. Pat. Nos. 3,448,302; 4,845,533; 4,876,667; 6,044,008.
Wire crossbar memory (MWCM) has also been proposed. See U.S. Pat. Nos. 6,128,214; 6,159,620; 6,198,655. These memory proposals envision molecules as bistable switches. Two wires (either a metal or semiconducting type) have a layer of molecules or molecule compounds sandwiched in between. Chemical assembly and electrochemical oxidation or reduction are used to generate an “on” or “off” state. This form of memory requires highly specialized wire junctions and may not retain nonvolatility owing to the inherent instability found in redox processes.
Recently, memory devices have been proposed which use nanoscopic wires, such as single-walled carbon nanotubes, to form crossbar junctions to serve as memory cells. See WO 01/03208 (“Nanoscopic Wire-Based Devices, Arrays, and Methods of Their Manufacture”), and Thomas Rueckes et al., “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing,” Science, vol. 289, pp. 94-97 (2000). Hereinafter these devices are called nanotube wire crossbar memories (NTWCMs). Under these proposals, individual single-walled nanotube wires suspended over other wires define memory cells. Electrical signals are written to one or both wires to cause them to physically attract or repel relative to one another. Each physical state (i.e., attracted or repelled wires) corresponds to an electrical state. Repelled wires are an open circuit junction. Attracted wires are a closed state forming a rectifying junction. When electrical power is removed from the junction, the wires retain their physical (and thus electrical) state thereby forming a nonvolatile memory cell.
The NTWCM proposals to date rely on directed growth or chemical self-assembly techniques to grow the individual nanotubes needed for the memory cells. These techniques are now believed to be difficult to employ at commercial scales using modem technology. Moreover, they may contain inherent limitations such as the length of the nanotubes that may be grown reliably using these techniques, and it may be difficult to control the statistical variance of geometries of nanotube wires so grown.